The present invention relates to a drain recovery system for the condensate feedwater system of a nuclear power plant and, more particularly, to a drain recovery system in which drains from feedwater heaters of the condensate feedwater system is collected and recovered to the condensate feedwater system.
Hitherto, a drain recovery system for the condensate feedwater system of a nuclear power plant has been known in which the feedwater heater drains stored in a drain tank are pumped-up by drain pumps to be injected into the condensate feedwater system at a predetermined portion thereof for recovery of the drains. For instance, "Technical Report of Mitsubishi Heavy Industries", page 16, vol. 17 (published March, 1980) discloses such a system adapted to recover a high-pressure feedwater heater drain in a pressurized water reactor (PWR) plant. On the other hand, boiling water reactor (BWR) plants operating in the U.S.A., such as BRUNSWICK Nos. 1 and 2 and Grand Gulf No. 1 each incorporate therein also such a systems adapted to recover a high-pressure feedwater drain.
In these known drain pumping-up recovery systems, the pressure of the portion of the condensate feedwater system at which the feedwater heater drain is injected thereinto varies significantly depending on the level of the load of the plant. More specifically, when the plant load is comparatively in a low level, the flow rate of the feedwater is correspondingly small, which in turn reduces the pressure drop in the condensate feedwater system and also serves to increase the delivery head of the condensate pumps, so that the pressure in the portion of the condensate feedwater system at which the collected drain is injected thereinto is increased. This means that the required delivery head of the drain pumps is largely changed depending on the level of the load. Namely, delivery head required for the drain pumps is large at the low load level and small at the high load level, so that the drain pumps must satisfy a wide range of the required delivery head. This in turn requires drain pumps of a large capacity, and complicates the specification and design of the drain pumps.
On the other hand, in case of the BWR plant, it is necessary to maintain the purity of the water in the condensate feedwater system at a degree as high as possible. This requires that the drain pumping-up recovery system incorporates a water purifier through which the drain is purified before it is injected into the condensate feedwater system. Unfortunately, however, the provision of the water purifier increases the pressure drop, which necessitates a further increase in the drain pump capacity and further complication of the design of the drain pumps. It might be desired that not only the high-pressure feedwater heater drain but also the low-pressure feedwater heater drain, are pumped-up and injected into the condensate feedwater system. In such a case, however, the flow rate of the drain treated by the drain pumps is increased and, in addition, the use of the water purifier becomes essential. This further increases the drain pump capacity so that the design of the pump is extremely complicated. For these reasons, the conventional system is not provided with a water purifier and is not designed for recovery of the low-pressure feedwater heater drain.
The known drain pumping-up recovery system encounters a problem that, when the drain pumps malfunction and trip, the flow rate of the drain injected into the condensate feedwater system and, hence, the flow rate of the feedwater supplied to the nuclear reactor becomes insufficient, and thus the plant might be scramed as a whole. In addition, the drain level in the drain tank is raised so that the flow of the drain from the high-pressure feedwater heater into the drain tank is impeded to cause a risk of a rise in the drain level in the high-pressure feedwater heater, which in turn may cause a reverse flow of the drains into the high-pressure turbine, with a result that the turbine is damaged. In order to obviate this problem, it has been necessary to install a spare drain pump and to start this spare drain pump in the event of a trip of one of the drain pumps. This also complicates the design of the drain pumps.